AD8230: input bias current over temperature

Q

The specifications for the AD8230 INA gives typical bias current of 150pA, max 1nA at 25degC. Can you tell me how this varies over the specified temperature range? I cannot find a graph of this or a 'max val over temp' specified.

I have a question also about figure 17 on rev B of the datasheet. It shows how Average Bias current varies with input CMV for various temperatures. The units of figure 17 (ds revB) is in uA. Shouldn't this be nA?

A

Since we don’t specify input bias variation over temperature for this part, I have very limited data. The good news is that most of this bias current is related to charging a sampling capacitor at the output, which barely changes over temperature. This means that the input bias remains below 10nA when common-mode voltages are close to zero (for split supplies).

However, this does not hold true for common-mode voltages other than zero. The sensitivity of the bias current to common-mode voltage variations is much larger than to temperature, which leads me to your second question. Yes, the plot is correct, the scale is in uA. This is, once again, due to the nature of the charging of the sampling capacitor, which is reset every cycle. This is why the behavior looks linear, as it behaves as a switched-capacitor circuit rather than a high-input impedance gate, like regular amplifier inputs.

In conclusion, you can get to this conclusion from observing figure 17. Notice how the input bias current lines cross around zero, suggesting very little variation in input bias over temperature when Vcm=0. The input bias variation over temperature also increases with Vcm.